= Solution
During <isovolumetric contraction>, ventricular <pressure> lies between the venous and arterial pressures, $p_v<p_{LV}<p_a$. Both valves are closed, so $Q_v=Q_a=0$ and ventricular volume remains at its end-filling value $V^+$. Consequently
$$
\boxed{p_{LV}=E_{LV}(t)(V^+-V_0).}
$$
Here $V^+$ is end-diastolic ventricular volume and $V_0$ is the zero-<pressure>/unstressed volume.
The arterial and venous reservoirs still exchange systemic <blood>:
$$
\dot p_a=-\frac{p_a-p_v}{R_cC_a},\qquad\dot p_v=\frac{p_a-p_v}{R_cC_v}.
$$
If contraction is short compared with both $R_cC_a$ and $R_cC_v$, the fractional changes are small and
$$
\boxed{p_a\simeq p_a^+,\qquad p_v\simeq p_v^+,}
$$
where the superscript values refer to the beginning of contraction. Thus <pressure> can rise rapidly in the <left ventricle> through increasing elastance while the other pressures are approximately fixed. This stage ends when $p_{LV}$ reaches $p_a$ and the <aortic valve> opens.
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